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Carbon arranges itself in startlingly different ways. The same six-proton atom can become diamond, the hardest substance we commonly handle, or graphite, soft enough to leave its name on a page. The difference is not in the atoms but in how they hold hands.

Diamond uses every bond. Each carbon meets four others in tetrahedra, making a lattice that does not negotiate. Graphite by contrast uses three, leaving one electron free to drift through stacked sheets — the reason a pencil writes and a battery conducts. The free hand is the difference.

Fullerenes close the sheet into a cage. Sixty atoms, twelve pentagons, twenty hexagons, and the molecule rolls. From the outside it looks like a small piece of geometry; from the inside, a hollow large enough to keep another atom prisoner.

Pull a single layer off graphite — no machine, just persistent tape, in the original Manchester experiment — and you have graphene. The thinnest material we know how to handle, and one of the strongest.

Each carbon is bonded to three others in flat hexagons, leaving a fourth electron free in a delocalized π-cloud above and below the sheet. This is why graphene conducts as well as it does, and why it bends without breaking.

σ ≈ 10⁸ S/m       E ≈ 1 TPa       d_C-C ≈ 0.142 nm

At room temperature, the electrons behave as if they have no mass — they move in straight lines until something stops them. This makes graphene a useful place to study relativistic physics on a tabletop.

In 1985 Kroto, Curl and Smalley vaporized graphite in a helium beam and discovered C₆₀ — sixty carbons assembled in the pattern of a soccer ball. Buckminsterfullerene, named for an architect who would have liked the joke.

Larger fullerenes — C₇₀, C₈₀, C₂₄₀ — are stable, and a few will accept a foreign atom into their interior. These are called endohedral fullerenes, and they remain mainly the property of curiosity. The interior is a quiet place. Nothing reacts there.

The largest carbon reservoir on Earth is the ocean. Not the coal, not the forests, not the atmosphere — the cold, slow water, which dissolves CO₂ and locks it into bicarbonate, carbonate, and the calcium shells of small animals.

Coral builds itself from carbon. So do the foraminifera that drift through sunlit water and, when they die, fall as pale rain to the seafloor — marine snow, the patient export. Over millions of years this becomes chalk, limestone, marble. The cliffs of Dover are mostly carbon from a vanished sea.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻

The fast carbon cycle — leaves, lungs, fires — completes in years. The slow cycle takes between a hundred thousand and a hundred million, moving carbon between rock, ocean, and air through volcanism and the weathering of stone.

Read patiently. The wiki is in no hurry, and neither is the carbon. Most of what is written here will, in some form, still be true a thousand years from now.

— end of vol. 炭, ed. 1